Ion migration barrier

ABSTRACT

An ion migration barrier for a circuit configuration (like a thermal print head) including a plurality of resistor elements which are selectively energized to produce localized areas of heat. When the circuit configuration is used in an environment including a source of ions, the ions tend to migrate towards the resistor elements, thereby shortening resistor element life. The ion migration barrier (which is an electrically conductive shield) isolates the resistor elements from the source of ions when it is positioned therebetween to thereby prolong resistor element life. In a circuit configuration like a thermal print head, the source of ions appears to be the thermally sensitive paper which is used with the print head.

United States Patent South Burlington, Vt.; John H. Hoskinson. Dayton. Ohio [21] Appl. No. 849,109 [22] Filed Aug. ll, I969 [45] Patented Aug. 10, 1971 [73] Assignee The National Cash Regkter Company Dayton,0hh

[54] [ON MIGRATION BARRIER 12 Claims, 4 Brewing Fl 52 us. Cl 219/216, 2 I'D/S43, 346/76 [51] lnt.Cl "05b "00 [50] FleldolSenrch 219/216. 543; 317/235; 346/76; 313/337 [56] Relerencs Cited UNITED STATES PATENTS 3,409,902 11/1968 Merryman.................... 219/216 X Inventors Rlchard C. CndyJr.

Primary Examinerl. V. Truhe Assistant Examiner-C. L. Albritton Arrorneys- Louis A. Kline. Albert L Sessler, Jr. and Elmer Wargo ABSTRACT: An ion migration barrier for a circuit configuration (like a thermal print head) including a plurality of resistor elements which are selectively energized to produce localized areas of heat. When the circuit configuration is used in an environment including a source ofions, the ions tend to migrate towards the resistor elements, thereby shortening resistor element life. The ion migration barrier (which is an electrically conductive shield) isolates the resistor elements from the source of ions when it is positioned therebetween to thereby prolong resistor element life. In a circuit configuration like a thermal print head, the source of ions appears to be the thermally sensitive paper which is used with the print head.

PATENTED AUG I 0 am SHEET 1 OF 2 INVENTORS RICHARD C. CADY JR. 8

JOHN H. HOSKINSON away 0 W .s a JN m A n A ION MIGRATION BARRIER BACKGROUND OF THE INVENTION This invention relates to an ion migration barrier for use in circuit configurations having planar resistors whose operating lives are shortened due to ions migrating towards one of the terminal ends thereof.

A typical application for this invention resides in the thermal print head art. One such print head is shown in copending U.S. Pat. application, Ser. No. 672,131, filed Oct. 2, I967, by Richard C. Cady and Robert M. Whitely, which application is assigned to the assignee of the present invention. Another thermal print head is shown in U.S. Pat. No. 3,l6l,457, which issued Dec. 15, 1964, on the application of Hans Schroeder et al.

A thermal print head of the type in which this invention may be used contains a plurality of resistor elements arranged on a substrate in the form of a printing matrix. The print head also includes circuit connection means for selectively energizing selected ones of said resistor elements to form desired characters to be printed. Those resistor elements which are selectively energized become heated, and, when the print head is brought into contact with a thermally sensitive paper, the desired characters are formed thereon.

A print head of the type described in the previous paragraph fails after printing about 5,000,000 lines when the print head is subjected to actual printing life tests. The heating values of the resistors used in the print head also change approximately +25 percent after 5,000,000 lines of printing. It appears as though sodium ions from the thermally sensitive paper migrate towards the negative ends of the resistors to shorten the lives thereof.

When the ion migration barrier of this invention is incorporated in a print head of the type just described, the life of the print head is considerably increased. Print heads employing applicants ion barrier operated without failure beyond the printing of 15,000,000 lines in actual life tests. During these tests, the heating values of the resistors used in the print heads varied less than 2 percent during the printing of over l5,000,000 lines, compared to the 25 percent variation experienced after the printing of 5,000,000 lines in print heads without applicants barrier.

SUMMARY OF THE INVENTION This invention relates to an ion migration barrier for use in circuit configurations having a plurality of resistor elements therein, which configurations are used in environments containing ions which tend to migrate towards the resistor elements and cause the failure thereof. The barrier is an electrically conduction shield which is positioned between the resistor elements and the source of ions to isolate the resistor elements from the source of ions and thereby prolong the operating life of the resistor elements.

BRIEF DESCRIPTION OF THE DRAWINGS FIGv I is a general view in perspective, and partly in cross section, of a circuit configuration having planar resistors and an ion migration barrier of this invention.

FIG. 2 is a cross-sectional view, in elevation, of the circuit configuration shown in FIG. I and is taken along line 2-2 thereof.

FIG. 3 is a schematic plan view of the circuit configuration shown in FIG. 1, showing details of the connecting means for the planar resistors.

FIG. 4 is a cross-sectional view, in elevation, of the circuit configuration shown in FIG. I and is taken along line 4-4 thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 is a general view, in perspective, showing a portion of a circuit configuration having planar resistor elements which are arranged in groups 12 to form a matrix for a thermal print head. The top surface 14 of the configuration 10 comes into contact with a conventional, thermally responsive printing paper ll (FIG. 2) when printing is to be effected, and selected groups I2 of the resistor elements are energized to produce discrete areas of heat to form the desired characters on the printing paper 11. Twenty-five groups 12 of resistor elements are arranged in a five-by-five matrix, as shown in FIG. 3, for the printing of a character.

The general construction of the circuit configuration 10, made by conventional printed circuit techniques, is shown in FIGS. 1 to 4 inclusive. The configuration 10 includes a base 16 (like glass), on which a layer 18 of tantalum oxide is deposited. A lamination composed of a first layer of tantalum, a layer of gold, and a second layer of tantalum is then deposited over the layer 18, and the lamination is masked and etched to produce individual planar resistor elements 20, 22, and 24 (made from the first layer of tantalum) for each group 12, and support elements 26, 28, 30, 32, and 34 (also made from the first layer of tantalum The support elements 26, 28, 30, 32, and 34 support conductor layers 26a, 28a, 30a, 32a, and 34a (made from the layer of gold), respectively, which are used in a connection pattern which is used to connect the resistor elements to a source of electrical potential (not shown).

The connection pattern for interconnecting the various resistor elements in the circuit configuration 10 is best shown in FIGS. 1 and 3. The gold layer 26a has a leg portion 36, which contacts one end of the resistor element 24 to make an electrical connection thereto. The remaining end of the resistor element 24 has a gold conductor strip 38 connected thereto to connect it with one end of the resistor element 22. A conductor strip 40, similar to the strip 38, is used to electrically connect the remaining end of the resistor element 22 with one end of the resistor element 20. The remaining end of the resistor element 20 is electrically connected to a gold layer 42 sup ported on an extension 44, of reduced width, of the resistor element 20. The resistor elements 20, 22, and 24 of each group 12 are in spaced, parallel relationship with one another and are series connected. The gold layer 260 may be connected to the negative terminal of a source of potential, and the gold layer 42 may be connected to its positive terminal through a conventional switching arrangement (not shown). The switching arrangement controls the selective energization of the particular groups 12 to effect the particular heating pattern desired to produce a specific character on the thermally responsive paper with which the configuration 10 may be used. The particular connection pattern used (that is, whether it is a common positive or a common negative) is of course de pendent upon the external circuitry with which the configuration It) is used.

The connection pattern described for one group 12 of resistor elements is generally the same for all groups 12 shown in FIG. 3. The groups 12 are slightly offset in the horizontal direction (as viewed in FIG. 3) to facilitate producing the connection pattern. Each of the gold layers like 260, 28a, 30a, 32a, and 34a has a thin layer 46 of tantalum deposited thereover, as shown in FIG. 2, and similarly the gold connector strips like 38 and 40 (FIG. I) also have a thin layer 48 deposited thereover to enable a protective layer of glass 50 to adhere thereto; these thin layers of tantalum come from the second layer of tantalum which is part of the lamination deposited on the layer 18. The layer of glass 50 also protects the various resistor elements 20, 22, and 24 of the groups 12.

The ion migration barrier of this invention is positioned in a circuit configuration between the source of ions and the resistor elements to be protected. In the specific circuit configuration 10 selected to portray the invention, it is suspected that the source of ions which migrate to the resistor elements, to curtail their useful lives, is the thermally sensitive paper with which the configuration I0 (which is a thermal print head in the illustrated embodiment) is used. The surface 14 of the configuration 10 contacts the thermally sensitive paper ll (FIG. 2), therefore, an ion migration barrier 52 is positioned between the surface 14 and the resistor elements of the groups 12. The barrier 52 is electrically conducting.

The ion barrier 52 (FIGS. 1, 2, and 4) may be made of several different conductors and semiconductors by a variety of conventional techniques. In the embodiment shown, the barrier 52 was made of chromium which was vapor deposited on the layer of glass 50. The barrier 52 has a thickness of about 500 to 2,000 Angstrom units. Any material selected for a barrier 52 should adhere well to the layer of glass 50, which should also be made thick enough to withstand the voltage between the barrier 52 and the underlying printed layers like 260, 38, and 40 and the resistor elements of the groups [2. The barrier 52 may be electrically biased positively or negatively with respect to the resistor elements 20, 22, and 24 of the groups 12 and their connection pattern. Leaving the barrier 52 electrically isolated also appears to have merit. The ion barrier 52 shown in FIGS. 1, 2, and 4 is actually one continuous layer extending across the entire configuration 10. In some cases, it may be desirable to have a separate shield (not shown) covering each group 12 of resistor elements. This separate shield is similar to the barrier 52 except that it is biased or pulsed with the same current which is used to energize the resistor elements of the particular group 12 of resistor elements with which the separate shield is associated.

When the circuit configuration (with the ion barrier 52) is used in a thermal printing environment, the positive sodium ions which apparently come from the thermally sensitive paper, as previously mentioned, seem to distribute themselves evenly over the top surface 54 (FIG. 2) of the barrier 52 instead of being attracted to the negative ends of the resistor elements 20, 22, and 24, where they would cause early failure of these elements. Without the barrier 52, the positive ions would penetrate the layer of glass 50 and move along the resistor elements of the groups 12 under the influence of electrical fields along the lengths of these elements, between them, and between the interconnection leads and the resistor elements and finally settle at the negative ends of the resistor elements to cause their early failure. An optional layer of glass 56 protects the barrier 52 from abrasion and damage.

The drawings depicting an embodiment of this invention are not drawn to scale, The printed resistor elements are approximately 600 Angstrom units thick, and the gold layers, like 260. 28a, etc.. are approximately 2,000 Angstrom units thick. There is no intent to place a deep well or cavity 58 (FIGS. 1 and 2) above the resistor elements of the groups 12. Any well which might exist is very shallow and has no visible effect on the printing operation when the circuit configuration 10 is used in a thermal-printing environment.

WHAT WE CLAIM IS:

I. A circuit having a plurality of resistor elements. which circuit is used in an environment including a source of ions which ions migrate towards said resistor elements to cause the failure thereof; and

an electrically conductive shield positioned between said resistor elements and said source of ions to isolate said ions from said resistor elements.

2. The circuit as claimed in claim 1 in which said resistors are planar and said shield is a thin layer of metal.

3. The circuit as claimed in claim 2 further comprising a layer of dielectric material positioned between said resistor elements and said shield.

4. The circuit as claimed in claim 1 in which said shield is electrically biased positively with respect to said resistor elements.

5. The circuit as claimed in claim I in which said shield is electrically biased negatively with respect to said resistor elements.

6. The circuit as claimed in claim I in which said shield is electrically isolated from said resistor elements.

7. A thermal print head having an external surface for contacting a record medium in printing engagement therewith comprising:

a plurality of resistor elements arranged on a substrate in the form of a printing matrix; circuit connection means for selectively energizing selected ones of said resistor elements to form characters using said matrix;

said print head being adapted for use in an environment in eluding a source of ions which ions migrate through said external surface towards said resistor elements to cause the deterioration thereof; and

an electrically conductive shield positioned between said external surface and said resistor elements to minimize deterioration of said resistor elements and prolong their useful lives.

8. The print head as claimed in claim 7 in which said shield is a thin layer of metal.

9. The print head as claimed in claim 8 in which said shield is separated from said resistor elements by a layer of dielectric material positioned therebetween.

10. The print head as claimed in claim 9 in which said shield has a layer of dielectric material covering the side of the shield positioned away from the resistor elements.

11. The combination comprising:

a thermal print head having an external surface for contacting a record medium in printing engagement therewith and having a plurality of resistor elements arranged on a substrate in the form ofa printing matrix;

circuit connection means for selectively energizing selected ones of said resistor elements to form characters using said matrix;

a thermally sensitive record medium contacting said external surface in printing engagement with said print head; and

an electrically conducting shield positioned between said resistor elements and said external surface to minimize deterioration of said resistor elements and prolong their useful lives.

12. The combination as claimed in claim 11 in which said shield is a thin layer of metal which is supported on a layer of dielectric material which is positioned between said shield and said resistor elements. 

1. A circuit having a plurality of resistor elements, which circuit is used in an environment including a source of ions which ions migrate towards said resistor elements to cause the failure thereof; and an electrically conductive shield positioned between said resistor elements and said source of ions to isolate said ions from said resistor elements.
 2. The circuit as claimed in claim 1 in which said resistors are planar and said shield is a thin layer of metal.
 3. The circuit as claimed in claim 2 further comprising a layer of dielectric material positioned between said resistor elements and said shield.
 4. The circuit as claimed in claim 1 in which said shield is electrically biased positively with respect to said resistor elements.
 5. The circuit as claimed in claim 1 in which said shield is electrically biased negatively with respect to said resistor elements.
 6. The circuit as claimed in claim 1 in which said shield is electrically isolated from said resistor elements.
 7. A thermal print head having an external surface for contacting a record medium in printing engagement therewith comprising: a plurality of resistor elements arranged on a substrate in the form of a printing matrix; circuit connection means for selectively energizing selected ones of said resistor elements to form characters using said matrix; said print head being adapted for use in an environment including a source of ions which ions migrate through said external surface towards said resistor elements to cause the deterioration thereof; and an electrically conductive shield positioned between said external surface and said resistor elements to minimize deterioration of said resistor elements and prolong their useful lives.
 8. The print head as claimed in claim 7 in which said shield is a thin layer of metal.
 9. The print head as claimed in claim 8 in which said shield is separated from said resistor elements by a layer of dielectric material positioned therebetween.
 10. The print head as claimed in claim 9 in which said shield has a layer of dielectric material covering the side of the shield positioned away from the resistor elements.
 11. The combination comprising: a thermal print head having an external surface for contacting a record medium in printing engagement therewith and having a plurality of resistor elements arranged on a substrate in the form of a printing matrix; circuit connection means for selectively energizing selected ones of said resistor elements to form characters using said matrix; a thermally sensitive record medium contacting said external surface in printing engagement with said print head; and an electrically conducting shield positioned between said resistor elements and said external surface to minimize deterioration of said resistor elements and prolong their useful lives.
 12. The combination as claimed in claim 11 in which said shield is a thin layer of metal which is supported on a layer of dielectric material which is positioned between said shield and said resistor elements. 